J. Nannoplankton Res. 37 (2–3), 2017, pp.177–188 © 2017 International Nannoplankton Association
ISSN 1210-8049 Printed by The Sheridan Press, USA

177

Biostratigraphic and morphometric analyses of specimensfrom the calcareous nannofossil genus Tribrachiatus
Jean M. Self-Trail*
U.S. Geological Survey, 12201 Sunrise Valley Dr., MS 926A, Reston, VA 20192, USA; *jstrail@usgs.gov

Ellen L. Seefelt
U.S. Geological Survey, 12201 Sunrise Valley Dr., MS 926A, Reston, VA 20192, USA

Claire L. Shepherd
GNS Science, Lower Hutt, New ZealandVictoria A. Martin
Northern Virginia Community College, Annandale, VA USA
Manuscript submitted 27th September, 2016; revised manuscript accepted 26th June, 2017

Abstract Biostratigraphic and morphometric analyses of calcareous nannofossil assemblages from one outcrop
and two cored sections of lower Eocene sediments reveal the presence of two new species: Tribrachiatus lunatus sp.
nov., and Tribrachiatus absidatus sp. nov. Differences between these new species and Tribrachiatus orthostylus are
discussed. The first occurrence of the two new species is just below the calcareous nannofossil Zone NP11/NP12
boundary, close to the Chron 24r/23n boundary, and thus they are globally useful biostratigraphic markers.

Keywords Calcareous nannofossils, biostratigraphy, early Eocene, taxonomy, Tribrachiatus
1. Introduction
From a recent study of lower Eocene sediments from theAtlantic Coastal Plain of Maryland (Self-Trail, 2011),
two new calcareous nannofossil species belonging to
the genus Tribrachiatus are identified. The history of thetaxonomy of the genus Tribrachiatus is somewhat convoluted and often hard to follow. Tribrachiatus orthostylus, the type species of the genus, was originally named
Discoaster tribrachiatus by Bramlette & Riedel (1954),
who described a large, triradiate discoaster with or without bifurcations and having slightly bent rays. This species was later used to establish the genus Tribrachiatus by
Shamrai (1963). Members of this genus were short-lived,
first appearing in the early Eocene (Zone NP10) near the
top of Chron 24r (Wei & Zhong, 1996) and having theirbiostratigraphic last occurrence (LO) within Zone NP12,
also in the early Eocene. Tribrachiatus is thought to have
evolved from Rhomboaster by rotation and flatteningof the nannolith, a process clearly illustrated by Wei &Zhong (1996; their fig. 3). Early forms of T. orthostylus(Type A of Wei & Zhong, 1996) are bifurcated (Pl. 3, figs 1,
4–6) and later forms are not.
Although Bramlette & Riedel (1954) stated in their
original description that many of these forms have slightly
bent rays, this feature is rarely documented, and most published light microscope images of T. orthostylus show flatarms (e.g. Wei & Zhong, 1996; Bybell & Self-Trail, 1997;
Bown, 2005). However, Bramlette & Sullivan (1961)
published one light micrograph (LM) image of a strongly

curved specimen, Perch-Nielsen (1977) published one
scanning electron micrograph image of a strongly curved
T. orthostylus from the South Atlantic, and Shepherd &Kulhanek (2016) documented the presence of two forms
from New Zealand (Morphotypes A and B) that show
curving of two of the three arms.
This paper documents the taxonomy and biostratigraphic ranges of two new species of Tribrachiatus in
sediments from the United States Atlantic Coastal Plainand New Zealand. The first occurrence (FO) of these
new forms is a biostratigraphically useful event in lower
Eocene marine sediments. Documentation of the absenceof bifurcations in early forms of T. orthostylus confirms
the report of Wei & Zhong (1996), and the extreme
curving of the arms in the two new species described inthis paper, T. lunatus sp. nov. and T. absidatus sp. nov.,
suggests that the early trend of slightly curving arms in
T. orthostylus continued and became more pronounced in
the late early Eocene.
2. Materials and methods
Specimens of Tribrachiatus analyzed and measured in thisstudy were taken from the Bass River (BR) core, locatedin the Atlantic Coastal Plain of New Jersey (39.6116°N,
74.4366°W), the South Dover Bridge (SDB) core, locatedin the mid-Atlantic Coastal Plain of Maryland (38.7469°N,
76.0069°W), and from the mid-Waipara River (MW) section located on the Pacific coast of the South Island of
New Zealand (43.0539°S, 172.6103°E) (Figure 1).

178

Self-Trail et al.

double slurry method of Watkins & Bergen (2003) andwere mounted using Norland Optical Adhesive 61. Slides
examined at the U.S. Geological Survey were scanned
using a Zeiss Axioplan 2 light microscope at 1250x magnification under cross-polarized (XPL), plane-polarized(PPL) and phase-contrast (PC) light. Slides examined
at GNS Science in New Zealand were scanned using an
Olympus BX53 microscope at 1000x magnification under
XPL, PC and plane-transmitted (PL) light. Scanning electron micrographs were taken at the U.S. Geological Survey using a Hitachi SU-5000 Field Emission scanningelectron microscope (SEM). Absolute abundance counts
were obtained by counting the number of Tribrachiatus
specimens per 450 specimens of calcareous nannofossils. To ensure that the biostratigraphic ranges of the new
Tribrachiatus specimens were accurate, one complete traverse of the short axis of each coverslip was scanned inaddition to the 450 specimens and any specimens noted
were added to the tally.
The computer program ImageJ, a free program
designed and distributed by the National Institute of
Health, was used to measure T. absidatus and T. lunatus.
These measurements include 1) the angle formed at thebend in the arm, as measured from a line drawn from thecenter of the specimen to the tip of the arm, herein designated as the arm angle (see arrow in Figure 2A, 2B);
2) the width between the tips of the two major arms
(Figure 2A, 2B); and 3) the outer circumference of a circle
transcribed around all three arms. In T. orthostylus, only
the outer circumference was measured (Figure 2C). These
measurements were used to help determine the variationin size and shape for each species and to determine exactly
how the two new species differ from each other and
T. orthostylus.

Figure 1: Map showing the location of the South Dover Bridge (SDB)
and Bass River (BR) cores (U.S. Atlantic Coastal Plain) and the midWaipara River (MW) section, South Island of New ZealandIn the SDB and BR sections, 105 and 133 samples
were taken from the Nanjemoy and Manasquan Formationsof early Eocene age, respectively. Samples were extracted
from the center of freshly broken core segments in order toavoid residual drilling mud contamination. Outcrop material
from the MW section is comprised of 30 samples collected
in 2012 from an ~11m succession of lower Eocene Ashley
Mudstone and six samples collected in 2007 from the basal
Ashley Mudstone. The two sample sets were correlatedusing foraminifera, dinocyst and nannofossil bioevents to
produce a composite section. Smear slides were preparedbased on the techniques of Bown & Young (1998) and the

3. Abundance and biostratigraphic range
Tribrachiatus orthostylus has a distinctly different range
than T. lunatus and T. absidatus (Figure 3). Globally, theFO of T. orthostylus occurs near the top of Zone NP10,
where it is often used as a proxy for the Zone NP10/NP11
boundary (Martini, 1971; Agnini et al., 2014); however,this boundary is truncated in all three studied sections. The
top of Zone NP10 is missing from the SDB core (SelfTrail, 2011; Self-Trail et al., 2012), as evidenced by theabsence of T. contortus and the presence of a change inlithology. Miller et al. (1998) recorded an unconformablesurface at the Zone NP10d/NP11 boundary in the Bass
River core based on the presence of rip-up clasts and achange in lithology. A more extensive unconformity is
documented from mid-Waipara, where upper Zone NP10
and lowermost Zone NP11 are missing (Shepherd &Kulhanek, 2016). This is clearly seen in Figure 3, where
all three species have concurrent first occurrences at MW
but not at SDB or BR.
The first rare appearance of T. absidatus, which occurs
just slightly before the FO of T. lunatus in both SDB andBiostratigraphic and morphometric analyses of specimens from the calcareous nannofossil genus Tribrachiatus

179

NP11/NP12 boundary, close to the C24r/C23n boundary(Shepherd & Kulhanek, 2016), and therefore these species
are useful biostratigraphic markers. The LO of all threeforms is simultaneous at all sites and approximates theZone NP12/NP13 boundary, as defined by Perch-Nielsen(1985). Although a minor unconformity is recorded at thisboundary in BR (Miller et al., 1998), it appears to be continuous at both SDB and MW (Self-Trail, 201l; Shepherd& Kulhanek, 2016).

w id
th (
μm
)

A Tribrachiatus lunatus

4. Morphometric analyses
Morphometric comparison between T. absidatus andT. lunatus confirms that the two are distinctly different
species, despite having nearly identical ranges. Comparison between the circumferences of all three species provides information on changing size through time.

arm angle

B Tribrachiatus absidatus
width (μm)

4.1 Arm angle (T. absidatus andT. lunatus)
The difference in arm angles between T. absidatus andT. lunatus is clearly delineated in Figure 4. The arm angle
in T. absidatus is somewhat variable, ranging between
95–141°. Specimens are rarely as perfect as the figured
holotype (Pl. 1, fig. 1), often having broken arm tips (Pl. 1,
fig. 4) or showing differing asymmetrical angles fromarm to arm (Pl. 1, fig. 8). The arm angle in T. absidatus
at Bass River is slightly bigger than in the other sections,with no recorded specimens below 110°. The arm angle
in T. lunatus is constrained between 135–160°, with only
one outlier in MW having an angle >160° (Figure 4). No
significant variation exists between populations from thethree cores or through time.

arm angle

C Tribrachiatus orthostylus
um

cir
c

ence
fer
Figure 2: Diagram showing measurement parameters: A) the arm angle
and tip-to-tip width of T. lunatus, B) the arm angle and tip-to-tip width ofT. absidatus, and C) the circumference of T. orthostylus

BR (Figure 3), is easily missed during routine scanning.
A single occurrence of T. absidatus in SDB below its
FO is due to mixing of sediments at a burrowed contact.
Both T. lunatus and T. absidatus first occur just below the

4.2 Tip-to-Tip width (T. absidatus andT. lunatus)
For T. absidatus, the tip-to-tip width of the arms ranges
between 6.33–22.4μm, with the majority at MW andSDB falling at or below 10μm (Figure 5). However, theclustering of specimens from Bass River to the right of
the 10μm line clearly shows that the specimens from BR
are, on average, slightly bigger than those from SDB and
MW. The range in the tip-to-tip width between arms in
T. lunatus is similar to the range for T. absidatus, from
6.9–15.6μm, with the majority falling between 8–12μm(Figure 5). It is interesting to note that specimens from
MW are smaller than those analyzed from the U.S. Atlantic Coastal Plain. They range between 6.9–10.2μm, with
only two outliers of 11.7μm.
4.3 Circumference
The circumference of T. orthostylus was measured bydrawing a perfect circle around the outside of each specimen, making sure that the circle touched all three arms
(Figure 2C). The circumference of specimens from SDB(except for two outliers) and MW is below 250μm, andaveraged 143μm and 110μm, respectively (see Figure 6).

180

Self-Trail et al.

Figure 3: Graphs showing the relative abundance and biostratigraphic range of T. orthostylus (red line), T. absidatus (green dashed line), and T. lunatus (blue dashed line). Shaded area is lowermost Zone NP11, which is missing at the mid-Waipara River section

However, specimens from the basal section of BR (Zone
NP11) are significantly bigger and have an average circumference of 287μm and an overall average size of
188μm. Bifurcated specimens from all three sites are typically in the larger size fraction.
For comparison purposes, the circumferences ofT. absidatus and T. lunatus were measured by drawing acircle that followed the outer edge of the two main arms.
In BR and SDB, T. absidatus averages 143μm and 130μm
respectively, and T. lunatus averages 132μm and 141μm,
respectively. However, specimens from MW are considerably smaller, averaging just 97μm for T. absidatus and89μm for T. lunatus (Figure 6).
5. Evolution and discussion
Wei & Zhong (1996) clearly documented the evolutionof the genus Rhomboaster to Tribrachiatus by the rotation and flattening of rhombic cubes through time tothree armed forms with bifurcated tips, and a tendencyof the arms to curve slightly downward (illustrated here
in Pl. 3, fig. 13; Pl. 4, figs 1–3). The continued curvature of two arms in the same plane produced a crescentshape in T. lunatus and a vaulted shape in T. absidatus,
while curvature out of the flat plane and simultaneous
reduction of the length of the third arm continued in both
species. This trend is recorded by both species in different ways: rounding of the whole body to a crescentshape in T. lunatus and bending of the arm tips by >95°
in T. absidatus. Although the angle between arms of
T. orthostylus must increase during the transition to
T. absidatus and T. lunatus, preliminary measurements ofspecimens from BR did not show any change. However,upper Zone NP10 sections are often very thin or lacking
on the Atlantic Coastal Plain (Gibson & Bybell, 1994;
Self-Trail, 2011) and it is probable that this section was
not sampled or is missing in BR.
The slightly longer third arm and sharply defined interior angle of T. absidatus, coupled with its slightly earlier
appearance in SDB and BR (see Figure 3), suggests thatT. absidatus evolved from T. orthostylus first, quickly followed by the evolution of T. lunatus. Although T. lunatus and T. absidatus have very similar ranges, Figure 4
illustrates the two are distinct species, as T. absidatus has
a consistently smaller average arm angle (<140°) than
T. lunatus (>140°).
It is interesting to note that specimens of T. lunatus,
T. absidatus and T. orthostylus in the MW section are
consistently smaller than at BR and SDB (Figures 5, 6).
This suggests either an intrinsic size difference between
the Atlantic and Pacific populations or a contributing paleoceanographic factor in the local formation of the coccoliths. Sediments from the MW section were deposited at
upper bathyal depths (Morgans et al., 2005; Hollis et al.,2012), while sediments from SDB and BR were deposited

Biostratigraphic and morphometric analyses of specimens from the calcareous nannofossil genus Tribrachiatus

181

-"A

-"?

-"O
P2-+

8?79# m?5(= :=!*%( *(>9# GAF%+ 3)2/*),%+ /-,!&/*),

-OB

-O+

-OA

NB

-BB

--B

-+B

-BB

-AB

-&B

-?B

-"B

NB

-BB

--B

-+B

-BB

-AB

-&B

-?B

-"B

NB

-BB

--B

-+B

-BB

-AB

-&B

-?B

-"B

BBB

B+B

P2-+

:C;; 9!5(= *(>9# GAF

B-B

BBB

P2-B

B&B

P2--

BAB

P2-+

-B

&

P2--

B

D&

D-B

P2-B

Q!*D4C!>C=C 9!5(= ;(,9!?@ GAF-&#=A #@%M( G3F

Figure 4: Comparison between the arm angles of T. absidatus (green
diamonds) and T. lunatus (blue squares). Note the clear separation ofangles between the two species. Solid vertical black line denotes the 140°
arm angle

Figure 5: Comparison between the tip-to-tip width of T. absidatus (green
diamonds) and T. lunatus (blue squares). Note that the width of specimensfrom the mid-Waipara River section is smaller than those from South Dover
Bridge and Bass River. The solid black vertical line denotes a width of 10μm

182

Self-Trail et al.

in mid- to outer-shelf environments, respectively, andwere relatively proximal to shore (Kopp et al., 2009; SelfTrail et al., 2012). Thus, factors relating to salinity, water
clarity and oxygenation could have imposed restrictions
on productivity and size.
Changing conditions through time also resulted in acorresponding change in size and shape. For example, theoldest specimens of T. orthostylus in BR are considerably
bigger than the correlative specimens in SDB, but by Zone
NP12 time (~53Ma) the two populations are similar in size
(Figure 6). The absence of lower Zone NP11 sedimentsfrom MW makes size comparisons to the other two sites
impossible. Additionally, bifurcated T. orthostylus arecommon at the beginning of this species range, but disappear by mid-NP12 time.
6. Systematic Paleontology
All figured specimens and type species are stored in thecalcareous nannofossil laboratory at the U.S. Geological
Survey in Reston, Virginia, USA. All light micrographspecimens were taken at the same magnification (x2000).
Scanning electron micrograph image scale bars are 5μm
unless otherwise noted. Nannolith shape terminology follows the guidelines of Young et al. (1997).
Family RHOMBOASTERACEAE Bown, 2005

Figure 6: Comparison between the circumference of T. absidatus (green
diamonds), T. lunatus (blue squares) and T. orthostylus (black circles)in South Dover Bridge, Bass River and mid-Waipara River. Large redcircles show specimens with bifurcations. Note that these occur in the
deepest part of the section at each site, with one exception at SDB. The
relative paucity of bifurcated specimens at mid-Waipara River is due toan unconformity at the base of NP11

Tribrachiatus absidatus sp. nov.
Pl. 1, figs 1–14
2016 Tribrachiatus Morphotype B, Shepherd & Kulhanek,
Pl. 11, figs 21–22
Derivation of name: From the Latin for “vaulted”, a reference to its resemblance to the vaulted interior of a classical building such as a cathedral.
Description: A medium to large, triradiate nannolithhaving two straight to slightly curved arms of equal lengththat exhibit a 95–140° bend near their pointed tips, and asmaller third arm that is less than half the length of the twomain arms. The interior angle is between 110–140°. Width
between ray tips is typically <15μm (Figure 5). Specimensfrom all three locations are commonly overgrown andexhibit high-order birefringence colors.
Differentiation: Tribrachiatus absidatus exhibits a
broad range of morphological variation, and overgrowth
can make it difficult to distinguish it from broken specimens of T. orthostylus and T. lunatus. Tribrachiatus absidatus can be differentiated from T. lunatus by the presenceof a vaulted interior angle rather than a crescent arc and by
the presence of bent tips at the end of the two main arms.
Arm tips are prone to breaking off (Pl. 1, fig. 4). Typically,
the third arm of T. absidatus is slightly longer than thethird arm of T. lunatus (e.g. Pl. 1, fig. 9).
Dimensions: The tip-to-tip width between the twomain arms of T. absidatus ranges from 6.33–22.49μm andthe interior angle ranges between 94–151° (n=126).
Holotype: Pl. 1, fig. 1

Biostratigraphic and morphometric analyses of specimens from the calcareous nannofossil genus Tribrachiatus

183

Paratype: Pl. 1, fig. 2
Type locality: South Dover Bridge core, Maryland,
USAType level: lower Eocene, Sample N13869, 180.35m(Zone NP12)
Occurrence: Zones NP 11–12

Remarks: Tribrachiatus orthostylus is a triradiate formshowing low order birefringence colors and having bifurcations at the tips of the arms in older specimens (Pl. 3,
figs 1, 3–6). The circumference has a range of 42–508μm;
larger specimens are recorded from BR (Figure 6).
Occurrence: upper Zone NP10-NP12

Tribrachiatus lunatus sp. nov.
Pl. 2, figs 1–14; Pl. 4, figs 10, 14–15

Acknowledgements
We thank Jim Browning, Rutgers Core Repository,
for access to the Bass River core, and Brett Valentine(USGS) for valuable assistance with the SEM. Ric Jordan(Yamagata University) provided invaluable technicaladvice on systematics. An early draft of the manuscript
benefitted from the reviews of Laurel Bybell and Marci
Robinson (both of USGS). The authors wish to thankDavid Bord (ALS) and Richard Howe (Chevron) for theirthoughtful reviews. Funding for this paper was provided inpart by the National Cooperative Geologic Mapping andthe Climate and Land Use Change Programs (USGS), andby the New Zealand Government through the Marsden Fund.
Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the
U.S. Government.

1961 Discoaster tribrachiatus Bramlette & Sullivan,
Pl. 13, fig. 11
1977 Tribrachiatus orthostylus Perch-Nielsen, Pl. 14, fig. 7
2016 Tribrachiatus morphotype A, Shepherd & Kulhanek,
Pl. 11, fig. 20
Derivation of name: From the Latin for crescent shaped,
a reference to its resemblance to a crescent moon.
Description: Medium to large, triradiate nannolithwith two curved arms of equal length and a third arm that
forms a small nub, occasionally curved, that lies midway
between the two other arms. Ray tips of the two main arms
typically taper to a sharp point and width between ray tips
is <15μm (Figure 5). When oriented so that the small
third arm is facing upward towards the lens, it has brightbirefringence colors (often green or pink), and the specimen looks like a marquise diamond ring (Pl. 2, fig. 13).
The crescent shape in larger specimens is somewhat flattened when compared to smaller specimens, which are
more rounded. Specimens from all sections are commonly
overgrown and exhibit high-order birefringence colors(Pl. 2, fig. 13).
Differentiation: Tribrachiatus lunatus differs fromT. absidatus by having a rounded interior that is crescentshaped and a smaller third arm. Additionally, T. lunatus
lacks the bent arm tips that are present in T. absidatus. It
can sometimes be difficult to distinguish between overgrown specimens of T. lunatus and T. absidatus using XPL(e.g., compare specimens of T. lunatus [Pl. 2, fig. 10-a]with Pl. 1, fig. 11-a), but is easier using PC (compare Pl. 2,
fig. 10-b to Pl. 1, fig. 11-b). Although curved specimens of
T. orthostylus can sometimes be mistaken for T. lunatus
if viewed from an odd angle, T. orthostylus typically has amuch lower order birefringence color than does T. lunatus
and is usually gray in color.
Dimensions: The tip-to-tip width between the twomain arms of T. lunatus has a range of 6.9–15.6μm (n=92).
Holotype: Pl. 2, fig. 1
Paratype: Pl. 2, fig. 4Type locality: mid-Waipara River section, Canterbury
Basin, New ZealandType level: lower Eocene, Sample F41309, 7.36m(Zone NP12)
Occurrence: Zones NP 11–12
Tribrachiatus orthostylus Shamrai, 1963
Pl. 3, figs 1–15; Pl. 4, figs 1–9, 11–13

References

Agnini, C., Fornaciari, E., Raffi, I., Catanzariti, R., Palike, H.,
Backman, J. & Rio, D. 2014. Biozonation and biochronology of Paleogene calcareous nannofossils from low
and middle latitudes. Newslettters on Stratigraphy, 47(2):131–181.
Bown, P.R. 2005. Palaeogene calcareous nannofossils from theKilwa and Lindi areas of coastal Tanzania (Tanzania Drilling
Project 2003–4). Journal of Nannoplankton Research, 27(1):21–96.
Bown, P.R. & Young, J.R. 1998. Techniques. In: P.R. Bown (Ed.).
Calcareous Nannofossil Biostratigraphy. Kluwer Academic
Press: 16–28.
Bramlette, M.N. & Riedel, W.R. 1954. Stratigraphic value of
Discoasters and some other microfossils related to recent
Coccolithophores. Journal of Paleontology, 28(4): 385–403.
Bramlette, M.N. & Sullivan, F.R. 1961. Coccolithophorids andrelated Nannoplankton of the early Tertiary in California.
Micropaleontology, 7(2): 129–188.
Bybell, L.M. & Self-Trail, J.M. 1997. Late Paleocene and early
Eocene calcareous nannofossils from three boreholes in an
onshore-offshore transect from New Jersey to the Atlantic
Continental rise. Proceedings of the ODP, Scientific Results,
150X: 91–110.
Gibson, T.G. & Bybell, L.M. 1994. Paleogene stratigraphy of
the Solomons Island, Maryland corehole. U.S. Geological
Survey Open-file Report, 94–708: 39pp.
Hollis, C.J., Taylor, K.W.R., Handley, L., Pancost, R.D., Huber, M.,
Creech, J.B., et al. 2012. Early Paleogene temperature
history of the Southwest Pacific Ocean: Reconciling proxies
and models. Earth and Planetary Science Letters, 349–350:
53–66. doi: 10.1016/j.epsl.2012.06.024.

184

Self-Trail et al.

Kopp, R.E., Schumann, D., Raub, T.D., Powars, D.S., Godfrey, L.V.,
Swanson-Hysell, N.L., Maloof, A.C. & Vali, H. 2009.
An Appalachian Amazon? Magnetofossil evidence forthe development of a tropic river-like system in the
mid-Atlantic United States during the Paleocene-Eocene
thermal maximum. Paleoceanography, 24: PA4211, doi:
10.1029/2009PA001783.
Martini, E. 1971. Standard Tertiary and Quaternary calcareous
nannoplankton zonation, In: A. Farinacci (Ed.). Proceedingsof the Second Planktonic Conference, Roma, 1970, Edizioni
Tecnoscienza, Rome, 2: 739–785.
Miller, K.G., Sugarman, P.J., Browning, J.V., et al., 1998. Bass
River Site. Proceedings of the ODP, Initial Reports, 174AX:
1–39.
Morgans, H.E.G., Jones, C.M., Crouch, E.M., Field, B.D., Raine,
J.I., Strong, C.P., et al. 2005. Upper Cretaceous to Eocene
stratigraphy and sample collections, mid-Waipara River section, North Canterbury. Institute of Geological & Nuclear
Sciences report 2003/08.
Perch-Nielsen, K. 1977. Albian to Pleistocene calcareous nannofossils from the western South Atlantic, DSDP Leg 39.
Proceedings of the ODP, Initial Reports, 39: 699–823.
Perch-Nielsen, K. 1985. Cenozoic calcareous nannofossils. In:
H. Bolli, K. Perch-Nielsen & J.B. Saunders (Eds). Plankton
Stratigraphy. Cambridge University Press, Cambridge:
427–554.

Self-Trail, J.M. 2011. Paleogene calcareous nannofossils ofSouthern Maryland, South Dover Bridge core, USA. Journalof Nannoplankton Research, 32: 1–28.
Self-Trail, J.M., Powars, D.S., Watkins, D.K. & Wandless, G.A.
2012. Calcareous nannofossil assemblage changes acrossthe Paleocene-Eocene Thermal Maximum: Evidence from a
shelf setting. Marine Micropaleontology, 92–93: 61–80.
Shamrai, I.A. 1963. Certain forms of Upper Cretaceous andPaleogene coccoliths and discoasters from the southern
Russian Platform, Izvestiya Vysshith Uchebnyk Zavedenii
Geologiaya i Razvedka, 6: 27–40.
Shepherd, C.L. & Kulhanek, D.K. 2016. Eocene nannofossil biostratigraphy of the mid-Waipara River section, Canterbury
Basin, New Zealand. Journal of Nannoplankton Research,
36(1): 33–59.
Watkins, D.K., & Bergen, J.A., 2003. Late Albian adaptive
radiation in the calcareous nannofossil genus Eiffellithus.
Micropaleontology, 49(3): 231–251.
Wei, W. & Zhong, S. 1996. Taxonomy and magnetobiochronology of Tribrachiatus and Rhomboaster, two genera of
calcareous nannofossils. Journal of Paleontology, 70(1):7–22.
Young, J.R., Bergen, J.A., Bown, P.R., Burnett, J.A., Fiorentino,A., Jordan, R.W., Kleijne, A., van Niel, B.E., et al. 1997.
Guidelines for coccolith and calcareous nannofossil terminology. Palaeontology, 40(4): 875–912.

Biostratigraphic and morphometric analyses of specimens from the calcareous nannofossil genus Tribrachiatus

185

Plate 1. Tribrachiatus absidatus

Illustrated specimens in XPL, PPL, and PC. Specimens are identified by section and depth. BR = Bass River; SDB = South Dover Bridge;
WR = mid-Waipara River

186

Self-Trail et al.

Plate 2. Tribrachiatus lunatus

Illustrated specimens in XPL, PPL, and PC. Specimens are identified by section and depth. BR = Bass River; SDB = South Dover Bridge;
WR = mid-Waipara River

Biostratigraphic and morphometric analyses of specimens from the calcareous nannofossil genus Tribrachiatus

187

Plate 3. Tribrachiatus orthostylus

Illustrated specimens in XPL, PPL, and PC. Specimens are identified by section and depth. BR = Bass River; SDB = South Dover Bridge;
WR = mid-Waipara River

188

Self-Trail et al.

Plate 4. SEM images

Tribrachiatus lunatus and T. orthostylus. Specimens are identified by section and depth. SDB = South Dover Bridge. SEM image scale bars
are 5μm unless otherwise noted